1. Field of the Invention
The present invention relates generally to methods for fabricating optical waveguides, and more specifically to methods for fabricating plural waveguide sections in a single substrate.
2. Description of the Prior Art
Formation of waveguides in lithium niobate (LN) and similar optical materials is typically accomplished by one of two well-known processes: titanium indiffusion and annealed proton exchange (APE). The APE process is increasingly favored over titanium indiffusion for commercial manufacturing applications due in part to the high temperatures required to achieve waveguide formation by the titanium indiffusion process.
APE traditionally involves a first step of exposing selected regions of an LN substrate to an acidic medium (deemed the proton exchange step), followed by a second step of maintaining the LN substrate at an elevated temperature for a specified time period (deemed the annealing step).
The physical and operational characteristics of waveguides fabricated by the APE process may be optimized for a particular application by tuning the process parameters. For conventional APE, the process parameters consist of the following:
Channel width (w)
Channel duty cycle (xcex7)
Proton exchange time (te)
Proton exchange temperature (Te)
Exchange agent
Anneal time (ta)
Anneal temperature (Ta)
It is noted that not all of the foregoing process parameters are independent, and that some of the parameters may not be easily varied. For example, the exchange agent (the acidic medium selected to effect proton exchange), which controls overall proton exchange rate at a given temperature, is generally considered to be a fixed parameter, due to the limited availability of acidic media which do not produce etching of the LN substrate.
It is further noted that the proton exchange time and temperature parameters te and Te are mutually dependent, i.e., one can control exchange depth by adjusting either process time or temperature. Similarly, the anneal time and temperature ta and Ta are mutually dependent. Because of these dependencies, the process temperatures Te and Ta are generally considered to be fixed, and only the process times are varied.
The limitations and dependencies discussed above effectively reduces the total number of independent APE process parameters to four: w, xcex7, te, and ta (it is recognized that there exists a weak interdependence between w and xcex7; however, this weak interdependence may be ignored for the purpose of this discussion). The designer of the waveguide-containing device may thus select appropriate values of channel width and duty cycle (which are controlled by adjusting the shape and dimensions of the mask defining the regions exposed to the acidic medium) and exchange and anneal times in order to produce a waveguide having desired physical and operational characteristics.
A problem arises in cases where an integrated optical component designer wishes to fabricate two or more waveguide structures having differing physical or operational characteristics in the LN substrate. In conventional APE waveguide fabrication, all waveguide structures are simultaneously formed in the LN substrate, i.e., a single APE process is employed. Due to the relatively limited number of independent process parameters that may be adjusted, it may be difficult or impossible to select a single set of process parameters that produce the desired physical and operational characteristics in all of the waveguide structures. In other words, a single-stage APE process does not offer a sufficient number of degrees of freedom to optimize fabrication of plural waveguide structures having disparate properties.
For example, a three-section coupler for use in a difference-frequency mixing application may include an input waveguide structure, having a relatively narrow channel width, coupled via an adiabatic taper structure to a multimode mixing waveguide structure having a relatively broad channel width. If the coupler is formed by conventional APE, the anneal time ta is set by the requirements of the multimode mixing (wide-channel) waveguide structure, and is consequently very long. This long ta results in excessive proton diffusion in the narrow-channel input waveguide structure, causing the mode size propagating therein to be relatively large. This condition is undesirable, as it prevents matching of the input waveguide mode to a standard optical fiber mode and thereby complicates the task of launching light into the three-section coupler.
U.S. Pat. No. 5,982,964 to Marx et al. describes one approach for creating additional degrees of process freedom to enable separate optimization of the characteristics of different waveguide structures formed in a common substrate. Marx et al. discloses fabricating a first waveguide structure by the titanium indiffusion process, which, as alluded to above, requires high-temperature conditions (approximately 1000xc2x0 C.) to enable titanium diffusion into the substrate to occur at an industrially practical rate. The titanium-indiffusion process is followed by fabrication of a second waveguide section by APE, which is performed at a relatively low exchange and anneal temperatures Te and Ta (typically 275xc2x0 C. and 400xc2x0 C., respectively). Because the titanium atoms possess very low mobility at the exchange and anneal temperatures, the first waveguide structure remains substantially unchanged during the APE process. In this manner, the parameters of the titanium-indiffusion and APE processes may be independently tuned to optimize desired characteristics in the first and second waveguide structures. It is noted, however, that the Marx et al. approach increases the complexity (and potentially the cost) of manufacture of integrated optical devices by requiring use of two different waveguide fabrication processes operating at different temperature ranges.
Roughly described, the invention provides a method for forming plural waveguide structures having separately optimized physical and optical characteristics in a common optical substrate. The method comprises a first APE stage, including a first proton exchange step and a first annealing step, wherein protons are diffused into a first region of the optical substrate corresponding to at least a first waveguide structure, and a second APE stage, including a second proton exchange step and a second annealing step, wherein protons are diffused into a second region of the substrate different from the first region and corresponding to at least a second waveguide structure. The first and second regions of the substrate may be defined by openings in first and second masks, which are deposited on the substrate and patterned using conventional techniques. A set of process parameters (e.g., time/temperature conditions, mask channel width, duty cycle, and exchange agent) is selected for each APE stage so as to obtain targeted optical and physical properties in the associated waveguide structure. In effect, the method expands the number of degrees of process freedom relative to a conventional single-stage APE process to thereby enable each waveguide structure to be independently optimized.
The multi-stage APE method of the present invention may be advantageously utilized to fabricate any number of high-performance, compact integrated optics devices, including without limitation sub-Rayleigh range couplers and surface step couplers.